Pure vs impure functions definition and example

Described below are the important distinctions of Pure and Impure functions.

Pure function

  • Returned value depends on the values of the arguments
  • Pure functions do not call other systems or applications such as network or database calls
  • They do not use other variables besides their arguments
  • They are independent and only calculate a new value from its arguments
  • Pure functions will always return the same result given the same arguments
  • they also do not modify the arguments
  • Returns a new object
// Pure functions
function double(x) {
    return x + x;

function doubleAll(list) {
    // given an array it will not modify the array it will return a new object
    return list.map(double);

Impure function

  • Impure functions call some external system or application such as a database or network call
  • They reach out to other properties of the system or application
  • Modify the arguments passed to function
  • Returns same object or modified version of same object
// Impure functions
function double(x) {
    return x + x;

function doubleAll(list) {
    for (let i = 0; i < list.length; i++) {
        list[i] = double(list[i]);
    return list;


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